Spatial Confinement as an Effective Strategy for Improving the Catalytic Selectivity in Acetylene Hydrogenation

Promotion Effect of Well-Defined Deposited Water Layer on Carbon Monoxide Oxidation Catalyzed by Single-Atom Alloys

Single-atom alloys (SAAs) exhibit excellent catalytic performance and unique electronic structures, emerging as promising catalysts for potential industrial reactions. While most of them have been widely employed under reducing conditions, few are applied in oxidation reactions. Herein, using density functional theory calculations and microkinetic simulations, we demonstrate that a well-defined one water layer can improve CO oxidation on model SAAs, with reaction rates increased by orders of magnitude. It is found that the formation of hydrogen bonds and the transfer of charges effectively enhance the adsorption and activation of oxygen molecules at the H₂O/SAA interfaces, which not only increases the surface coverage of O₂ species but also reduces the energy barrier of CO oxidation. The proposed strategy in this work would extend the application range of SAA catalysts to oxidation reactions.

Sulfidated microscale zero-valent iron/reduced graphene oxide composite (S-mZVI/rGO) for enhanced degradation of trichloroethylene: The role of hydrogen spillover

Atomic hydrogen (H*) has long been thought to play an important role in the dechlorination of trichloroethylene (TCE) by carbon-supported zero-valent iron (ZVI), which offers an alternative pathway for TCE dechlorination. Herein, we demonstrate that the reductive dechlorination of TCE by sulfidated microscale ZVI (S-mZVI) can be further enhanced by promoting the formation of H* through the introduction of reduced graphene oxide (rGO). The completely degradation of 10 mg/L TCE can be achieved by S-mZVI/rGO within 24 h, which was 3.3 times faster than that of S-mZVI. The change in the distribution of TCE degradation products over time suggests that the introduction of rGO leads to a change in the dechlorination pathway. The percentage of ethane in the final products of TCE degradation by S-mZVI/rGO was 34.3 %, while that of S-mZVI was only 21.9 %. The electrochemical tests confirmed the occurrence of hydrogen spillover in the S-mZVI/rGO composite, which promoted the reductive dechlorination of TCE by H*. Although the S-mZVI/rGO composite had stronger hydrogen evolution propensity than S-mZVI, the S-mZVI/rGO composite still exhibited higher electron utilization efficiency than S-mZVI thanks to the increased utilization of hydrogen.

Dynamic Construction and Maintenance of Confined Nanoregions via Hydrogen-Bond Networks between Acetylene Reactants and a Polyoxometalate-Based Metal-Organic Framework

The nanoconfinement effect in catalysis has attracted much attention because it provides a novel means of regulating the molecular properties and related reactions. Confined nanoregions composed of both reactants and catalysts through weak interactions are expected to improve the catalytic performance and promote the mass transport of relevant molecules simultaneously. However, at reaction temperatures, the structural variation of such confined spaces constructed via weak interactions remains unclear. Herein, through density functional theory calculations combined with ab initio molecular dynamics simulations, we have systematically investigated the dynamic structural evolution of the confined space constructed by acetylene reactants and a polyoxometalate-based metal-organic framework (POMOF) via hydrogen-bond networks. It is found that, at the reaction temperature of acetylene semihydrogenation, the hydrogen-bond networks and generated confined nanoregions are not rigid but are constantly changing and dynamically maintained. The steering role played by the O atoms at the surfaces of the polyoxometalate clusters is essential for generation of the hydrogen-bond networks and maintenance of the nanoregions. Upon confinement, the acetylene reactants can be better activated than those in an unconstrained atmosphere, which is reflected by the different dynamic distributions of the ∠CHC bending magnitude. Moreover, from a comparison of the distinct interaction characteristics between acetylene/ethylene and POMOF, the different manifestations in the adsorption energy variations of the confined molecules can be interpreted. This work helps to elucidate the underlying mechanisms of confined catalysis and may promote its application in practical catalytic processes.

Enhanced Crystallinity of Covalent Organic Frameworks Formed Under Physical Confinement by Exfoliated Graphene

The polymerization of 1,4-benzenediboronic acid (BDBA) on mica to form a covalent organic framework (COF-1) reveals a dramatic increase in crystallinity when physically confined by exfoliated graphene. COF-1 domains formed under graphene confinement are highly geometric in shape and on the order of square micrometers in size, while outside of the exfoliated flakes, the COF-1 does not exhibit long-range mesoscale structural order, according to atomic force microscopy imaging. Micro-Fourier transform infrared spectroscopy confirms the presence of COF-1 both outside and underneath the exfoliated graphene flakes, and density functional theory calculations predict that higher mobility and self-assembly are not causes of this higher degree of crystallinity for the confined COF-1 domains. The most likely origin of the confined COF-1's substantial increase in crystallinity is from enhanced dynamic covalent crystallization due to the water confined beneath the graphene flake.

Two-Dimensional Graphdiyne-Confined Platinum Catalyst for Hydrogen Evolution and Oxygen Reduction Reactions

Pt-based materials are the state-of-the-art catalysts for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR); however, there is still much room to improve the catalytic activity and enhance the stability of Pt-based catalysts. In this work, two-dimensional (2D) graphdiyne (GDY) with uniform distributed pores was applied to cover the Pt surface for catalyzing HER and ORR through density functional theory (DFT) calculations. The 2D confinement induced by GDY was found to improve the catalytic performance of the Pt catalyst from three aspects: (1) the 2D covering layer increases the stability of the Pt catalyst through forming the heterogeneous interface of GDY/Pt(111); (2) GDY/Pt(111) shows better catalytic activities of HER and ORR, with the smaller average overpotential values of 0.26 and 0.51 V, respectively, compared with those (0.29 V for HER, 0.62 V for ORR) on the Pt catalyst; (3) the confinement effect of GDY weakens the adsorption energy of CO to -1.81 eV (average value) from -2.14 eV on Pt(111), inhibiting CO poisoning. This work sheds new light on 2D confinement effects for HER and ORR, which opens up a new strategy for improving the catalytic performance of Pt-based catalysts.